Method and apparatus for cooling a CVI/CVD furnace

ABSTRACT

A method and apparatus for cooling a furnace configured for processing refractory composites comprising a cooling gas flowed in a closed circuit through the furnace and over a cooling element disposed within the furnace. The cooling gas can flow either by natural convection or a convective force.

RELATED APPLICATIONS

This non-provisional application claims the benefit of and incorporatesby reference, in its entirety, U.S. provisional application serial No.60/137,590, filed on Jun. 4, 1999. This application is related to U.S.Pat. No. 6,352,430 and application Ser. No. 09/780,170.

BACKGROUND OF THE INVENTION

The invention relates to method and an apparatus for cooling a furnaceconfigured for processing refractory composites. More specifically, theinvention is directed to method and apparatus for cooling a furnace morerapidly than known prior art methods.

Processing of refractory composites takes place at elevatedtemperatures. Such processing includes CVI/CVD deposition of a bindingmatrix within a fibrous preform structure, and heat treating refractorycomposites. According to conventional practice, the furnace is allowedto cool statically under vacuum or it is back-filled with an inert gassuch as nitrogen. Cooling the furnace to a low enough temperaturewherein the furnace may be opened can take days according to thismethod. In addition, cooling the furnace too rapidly or introducing areactive gas, such as oxygen, can cause damage to the furnace itself orthe refractory composites being processed in the furnace. Therefore, amethod and apparatus is designed whereby the furnace and the refractorycomposites are cooled more rapidly and at a controlled pace thanconventional known methods without causing damage to the composites.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the invention, a method is provided forcooling a furnace configured to process refractory composites,comprising the steps of: flowing a cooling gas in a closed circuitthrough the furnace, over the refractory composites disposed within thefurnace, and over a cooling element disposed within the furnace. Themethod according to the invention may further comprise the step offlowing the cooling gas by natural convection. The method according tothe invention may also further comprise the step of flowing the coolinggas by forced flow.

According to a further aspect of the invention, a furnace configured toprocess refractory composites and a cooling system therefor is provided,comprising: a furnace shell that defines a furnace volume; a heaterdisposed within the furnace shell; a cooling element disposed within thefurnace shell; an inlet conduit connected to the furnace shell in fluidcommunication with the furnace volume; an outlet conduit connected tothe furnace shell in fluid communication with the furnace volume; acooling gas supply configured to selectively introduce a cooling gasinto the furnace volume; and, a blower connected to the inlet conduitand the outlet conduit in fluid communication therewith, whereinactivation of the blower causes cooling gas introduced into the furnacevolume to flow through the blower, through the inlet conduit, over thecooling element, through the outlet conduit, and back to the blower in aclosed circuit.

The invention includes various other aspects as presented by thedetailed description that follows.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 presents a schematic view of a cooled furnace according to anaspect of the invention wherein the flow of cooling gas is induced bynatural convection.

FIG. 2 presents a schematic view of a cooled furnace according to anaspect of the invention wherein the cooling element is the furnace shelland the induction coil.

FIG. 3 presents a schematic view of a cooled furnace according to anaspect of the invention wherein the flow of cooling gas is forced.

FIG. 4 presents a schematic view of a cooled furnace according to anaspect of the invention wherein the flow of cooling gas is forced andwherein the inlet and outlet conduits are in an alternate position.

FIG. 5 presents a schematic view of a cooled furnace according to anaspect of the invention wherein the flow of cooling gas is forced andwherein the inlet and outlet conduits are in an alternate position.

FIG. 6 presents a schematic view of a cooled furnace according to anaspect of the invention wherein the flow of cooling gas is forced andwherein the reactive gas inlets are implemented to introduce a flow ofcooling gas.

FIG. 7 presents an embodiment of the invention wherein cooling gas isintroduced at multiple locations including the reactive gas inlets, andthe cooling element is the furnace shell and the induction coil.

FIG. 8 presents a cross-sectional view of a blower with an inert gaspurged dynamic shaft seal, according to an aspect of the invention.

FIG. 9 presents a side cross-sectional view of a furnace according to acertain embodiment of the invention.

FIG. 10 presents a side cross-sectional view of a furnace according to acertain embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Various aspects of the invention are presented in FIGS. 1-10, which arenot drawn to scale, and wherein like components, are numbered alike.Referring now to FIGS. 1-6, schematic representations of the basicconcepts according to certain aspects of the invention are nowpresented. Literal interpretation of the configurations presented inFIGS. 1-6 is not intended since the actual configuration may varygreatly depending upon the particular implementation into a specificfurnace design, all of which is within the skill of one of ordinaryskill in the art. The apparatus and method for cooling a CVI/CVDfurnace, as disclosed herein, may decrease the cooling time by about 25to about 50%.

Specifically referring now to FIG. 1, the method and apparatus arepresented for cooling a furnace 90 configured to process refractorycomposites, comprising the step of flowing a cooling gas 106 in a closedcircuit through the furnace 90 and over a cooling element 104 disposedwithin the furnace 90, as indicated by the flow path 94. The gas alsoflows over refractory composites 62 disposed inside the furnace 100. Asused herein, the term “refractory composites” includes fibrousrefractory articles fully or partially permeated with a bindingrefractory matrix, and intermediate refractory articles (refractoryfibrous preform structures, for example, such as carbon or ceramic fiberbrake disk preforms). The term “refractory composites” also includesporous substrates or structures which can be subject to a CVI/CVDprocess. A cooling medium, such as water, is circulated through thecooling element 104 and a heat exchanger 105 external to the furnace100.

The furnace 90 comprises a furnace shell 92 that defines a furnacevolume 114, and is disposed upon legs 113. A heating element 116 is alsodisposed within the furnace 90 and heats the refractory composites 62for CVI/CVD process and/or heat treatment processing. The heatingelement can be any heater. The gas flow is driven by natural convection.The refractory composites are quite hot at the beginning of the coolingprocess and heat the cooling gas to an elevated temperature which causesit to rise where it is cooled by the cooling element 104. The cooled gasfalls due to the force of gravity and is directed toward the outercircumference of the furnace and back up through the bottom where it isonce more heated. The cooling gas 106 is supplied to the furnace volume114 by a cooling gas supply 122 that may be comprised of a single gas ora plurality of individual different gas supplies 123 with individualflow quantities controlled by flow control valves 125. Any inert gas orcombination thereof can be used, provided that the gas or mixture ofgases have a high capacity and are stable under the operatingconditions. Examples of possible gases which can be used include, butare not limited to, nitrogen, helium or argon. Preferably, nitrogen isused.

Referring now to FIG. 2, a furnace 96 having a furnace shell 98 ispresented according to similar arrangement wherein natural convectionprovides the motive force for the cooling gas 106, as indicated by theflow path 99. In furnace 96 the heating element 116 comprises asusceptor 158 and an induction coil 160 disposed adjacent the susceptor158, and the cooling element is configured to cool the furnace shell 98,which, in this example, comprises a double wall with a space 97 inbetween filled with a cooling medium, such as cooling water that iscirculated through the heat exchanger 105. The space 97 may be separatedinto multiple sub-spaces with independent cooling water flow circuits.In addition, induction coils typically comprise a multitude of coilcooling passages 162 integrally formed into the induction coil 160.Thus, the cooling element may further comprise the induction coil 160with integral cooling passages 162 although, according to a preferredembodiment, the cooling is predominantly (if not totally) provided bythe furnace shell 98.

In the embodiments of FIGS. 1 and 2, openings may be provided throughthe various components and fixtures within the furnace to allow thecooling gas to flow in the manner described, or external conduits mayattached to the outside of the furnace to provide part of the flow pathoutside the furnace. These configurations, as well as other ones, arepossible as long as the apparatus provides the CVI/CVD furnace with thedesired cooling level.

Flow of the cooling gas 106 through the furnace may also be forced.Referring now to FIG. 3, a method is presented for cooling a furnace 100configured to process refractory composites, according to a furtheraspect of the invention, comprising the step of flowing a cooling gas106 in a closed circuit 102 through the furnace 100 and over a coolingelement 104 disposed within the furnace 100. The gas also flows overrefractory composites 62 disposed inside the furnace 100. A coolingmedium, such as water, is circulated through the cooling element 104 anda heat exchanger 105 external to the furnace 100.

According to a further aspect of the invention, a method is provided forcooling the furnace 100, comprising the step of flowing the cooling gas106 through the closed circuit 102 and over a cooling element disposedwithin the furnace, the closed circuit including the furnace 100, and ablower 108 disposed outside the furnace 100. The methods according tothe invention may further comprise the step of monitoring the oxygencontent of the cooling gas 106. An oxygen content analyzer 10 may beprovided that senses the oxygen content of the cooling gas 106 in theclosed circuit 102. The oxygen content is preferably maintained below apredetermined value so that oxidation does not occur in the furnace. Formost processes, oxygen content should be less than or equal to 100 ppmto avoid oxidation and problems associated therewith.

The invention is particularly useful for cooling furnaces used in hightemperature CVI/CVD and/or other heat treatment processes. As usedherein, the term “high temperature” means a temperature substantiallyelevated above room temperature in the range of 300° C. or greater.Refractory composites or materials, generally, are manufactured and/orprocessed at temperatures greater than 300° C., and may be at least 900°C. and on the order of 900°-3000° C., or higher. For example, a porouscarbon aircraft brake disk may have a pyrolytic carbon matrix depositedwithin it by a CVI/CVD process conducted at a temperature in the rangeof about 900°-1100° C., and may be further heat-treated at a temperatureup to 2200° C. or higher. Manufacturing and processing of other types ofceramic materials may occur at other temperatures.

Still referring to FIG. 3, a furnace and cooling system are presentedthat may be implemented in practicing the invention. According to anaspect of the invention, the combination is provided comprising afurnace 100, a cooling gas inlet 118 in fluid communication with thefurnace 100, a cooling gas outlet 120 in fluid communication with theCVI/CVD furnace 100, a cooling element 104 disposed within the furnace100 in a location where it may be exposed to cooling gas 106, and ablower 108 outside the furnace 100 connected to the cooling gas inlet118 and the cooling gas outlet 120 are provided. The blower 108 causescooling gas to flow in a closed circuit 102 through the cooling gasinlet 118, through the furnace 100 over the cooling element 104, andthrough the cooling gas outlet 120 back to the blower 108. According toa further aspect of the invention, the furnace 100 comprises a furnaceshell 112 that defines a furnace volume 114. The furnace shell may bedisposed upon legs 113. A heating element 116 and the cooling element104 are disposed within the furnace shell 112. The cooling gas inlet 118may be formed as an inlet conduit connected to the furnace shell 112 influid communication with the furnace volume 114. The cooling gas outlet120 may be formed as an outlet conduit also connected to the furnaceshell 112 in fluid communication with the furnace volume 114. A coolinggas supply 122 configured to selectively introduce the cooling gas 106into the furnace volume 114. The blower 108 is connected to the coolinggas inlet 118 and the outlet conduit 120 in fluid communicationtherewith. Activation of the blower causes cooling gas 106 introducedinto the furnace volume 114 to flow through the blower 108, through thecooling gas inlet 118, over the cooling element 104, through the coolinggas outlet 120, and back to the blower 108 in a closed circuit.

Although the cooling element 104 is shown at the top of the furnace inFIGS. 1-4, this position may not be the optimum position due to therelatively high temperatures typically encountered in that region. Aswill become apparent, the cooling element 104 may be placed in a varietyof positions within the furnace, and the inlet and outlet conduitpositions changed accordingly to cause the cooling gas to flow over thecooling element 104. In addition, the inlets and outlets may beconnected at multiple locations, as desired, to achieve a particularflow pattern.

The refractory composites 62 may comprise a multitude of poroussubstrates or structures stacked within the furnace 100 that are heatedand exposed to a reactant gas that breaks down and deposits a matrixwith the porous substrates 62. This process is commonly known aschemical vapor infiltration and deposition. Chemical vapor infiltrationand deposition (CVI/CVD) is a well known process for depositing abinding matrix within a porous structure. The term “chemical vapordeposition” (CVD) generally implies deposition of a surface coating, butthe term is also used to refer to infiltration and deposition of amatrix within a porous structure. As used herein, the term CVI/CVD isintended to refer to infiltration and deposition of a matrix within aporous structure. The technique is particularly suitable for fabricatinghigh temperature structural composites by depositing a carbonaceous orceramic matrix within a carbonaceous or ceramic porous structureresulting in very useful structures such as carbon/carbon aircraft brakedisks, and ceramic combustor or turbine components.

The generally known CVI/CVD processes may be classified into fourgeneral categories: isothermal, thermal gradient, pressure gradient, andpulsed flow. See W. V. Kotlensky, Deposition of Pyrolytic Carbon inPorous Solids, 8 CHEMISTRY AND PHYSICS OF CARBON, 173, 190-203 (1973);W. J. Lackey, Review, Status, and Future of the Chemical VaporInfiltration Process for Fabrication of Fiber-Reinforced CeramicComposites, CERAM. ENG. SCI. PROC. 10[7-8] 577, 577-81 (1989) (W. J.Lackey refers to the pressure gradient process as “isothermal forcedflow”). In an isothermal CVI/CVD process, a reactant gas passes around aheated porous structure at absolute pressures as low as a few torr. Thegas diffuses into the porous structure driven by concentration gradientsand cracks to deposit a binding matrix. This process is also known as“conventional” CVI/CVD. The porous structure is heated to a more or lessuniform temperature, hence the term “isothermal.” In a thermal gradientCVI/CVD process, a porous structure is heated in a manner that generatessteep thermal gradients that induce deposition in a desired portion ofthe porous structure. The thermal gradients may be induced by heatingonly one surface of a porous structure, for example by placing a porousstructure surface against a susceptor wall, and may be enhanced bycooling an opposing surface, for example by placing the opposing surfaceof the porous structure against a liquid cooled wall. Deposition of thebinding matrix progresses from the hot surface to the cold surface. In apressure gradient CVI/CVD process, the reactant gas is forced to flowthrough the porous structure by inducing a pressure gradient from onesurface of the porous structure to an opposing surface of the porousstructure. The flow rate of the reactant gas is greatly increasedrelative to the isothermal and thermal gradient processes which resultsin increased deposition rate of the binding matrix. This process is alsoknown as “forced-flow” CVI/CVD. Finally, pulsed flow involves rapidlyand cyclically filling and evacuating a chamber containing the heatedporous structure with the reactant gas. The cyclical action forces thereactant gas to infiltrate the porous structure and also forces removalof the cracked reactant gas by-products from the porous structure.Refractory composites are often subjected to heat treatments at varioustemperatures, and the invention is equally useful in furnaces employedfor that purpose. The furnace and fixture configuration may varysubstantially depending upon the type of process, and the variousaspects of the invention may be implemented with any of these processes,depending upon the particular configuration. As such, the furnaceconfigurations of FIGS. 1-7 and 9-10 are presented by way of example,and are not intended to limit the invention to the specific arrangementspresented as other variations or modifications that are evident topersons skilled in the art in light of the description provided herein.

According to a certain embodiment, the cooling gas 106 comprises apredetermined ratio of gases so long as the mixture of gases is stableat high temperatures and has the desired heat capacity. The cooling gassupply 122 may comprise a multitude of individual gas supplies 123 influid communication with the cooling gas inlet 118. Each individual gassupply 123 may provide a different gas composition, and flow controlvalves 125 may be provided to control flow of a particular gascomposition into the cooling gas inlet 118. The flow control valves 125may be used in combination to provide a flow of gas into the cooling gasinlet 118 comprising a predetermined ratio of gases by individuallycontrolling the flow of each gas. The cooling gas supply 122 may beconnected to the furnace 100 in other ways that introduce the flow ofcooling gas into the furnace, for example, by connecting the cooling gassupply 122 directly to the furnace 100, or by connecting the cooling gassupply 122 to the cooling gas outlet 120. Other alternatives forparticular applications are apparent to a person of ordinary skill inthe art in light of the description provided herein. The individual gassupplies 123 may be bottles or cylinders of gas or a gas supplyotherwise available at a manufacturing facility, such as for example butnot limited to, a plant nitrogen supply. Other suitable gases forcooling include helium and argon, typically supplied by bottle orcylinder. Although, nitrogen is relatively inexpensive, it may reactwith materials inside the furnace at elevated temperatures. For example,nitrogen may react with carbon/graphite above 2500° F. to form cyanogengas. Helium has a higher thermal conductivity than nitrogen or argon,but has a lower atomic weight than nitrogen or argon, so more isrequired. Argon is more stable than nitrogen at elevated temperatures,especially above 2500° F., has a much greater atomic weight than heliumand has a greater heat capacity than helium or nitrogen. An idealmixture takes advantage of all of these characteristics to provide theleast expensive mixture which is stable and has optimum coolingcharacteristics at the temperatures encountered for a particularprocess. The optimum mixture may be different for different processesand depends upon the peak temperatures encountered in the furnace, aswell as the possible reactivity of other components that are in thefurnace.

Alternatively, a single cooling gas may be employed. Any inert gas canbe used. If a single inert gas is used, nitrogen is preferred. If thecooling gas is reactive at a certain critical temperature, back-fillingthe furnace volume with the cooling gas may be delayed while the furnacecools under vacuum to a temperature less than the critical temperatureaccording to prior practice in the art. The cooling gas is subsequentlyintroduced into the furnace volume and circulated in the mannerdescribed. For example, if nitrogen is used as the cooling gas, thefurnace may be allowed to cool under vacuum according to prior practicein the art until reactive components are at a temperature on the orderof 2000° F. or less, after which the furnace volume is filled with thecooling gas to approximately atmospheric pressure and the cooling gas isthen circulated. The furnace volume may be partially filled if thetemperature is greater than the critical temperature, which may increasethe cooling rate with minimal chemical reaction. The temperature atwhich certain cooling gases are introduced may be dependent upon thereactivity of certain components within the furnace. The presence ofcertain cooling gases and the overall composition of the cooling gas maybe altered accordingly.

The composition of the cooling gas may be changed while it is beingcirculated in order to affect the rate at which the furnace is cooled.For example, the cooling rate typically decreases if the coolingconditions are not changed. Changing the cooling conditions may increaseor decrease the rate as a function of time. According to a certainembodiment, the composition of the cooling gas is changed to produce aconstant rate at which the furnace is cooled, which produces anapproximately linear time versus temperature curve (negative constantslope). The flow rate of the cooling gas may also be altered to affectthe rate at which the furnace is cooled, for example by increasing ordecreasing the rate. According to a certain embodiment, the flow rate ofthe cooling gas is altered to produce a constant rate at which thefurnace is cooled, which produces an approximately linear time versustemperature curve (negative constant slope). According to a preferredembodiment, both the gas composition and the cooling gas flow rate arechanged during the cooling process to produce a constant rate at whichthe furnace is cooled and a linear time versus temperature curve.

In the embodiment presented in FIG. 3, the furnace shell has two endportions 130 and 132, and the cooling gas inlet 118 is connected to oneof the end portion 130. The position of the cooling gas inlet 18 andcooling gas outlet 120 depends, in part, upon the desired flow patternof cooling gas through the furnace volume 114. As such, innumerablevariations are possible. Referring now to FIG. 2, for example, a CVI/CVDfurnace and cooling system is presented wherein the position of thecooling gas inlet 118 is moved to produce a change in the flow of thecooling gas. The various components previously described in relation toFIG. 3 are presented in FIG. 4, except that the furnace 100 is replacedby a furnace 124, having a furnace shell 126 with a center portion 128disposed between two end portions 130 and 132. According to this aspectof the invention, a closed circuit 134 having an inlet conduit 136 isconnected to the furnace 100 at the center portion 128. Connecting theinlet conduit 136 to the furnace 100 at the center portion 128 providesa flow of the cooling gas to the area that is typically the hottest.According to a further aspect of the invention, there is a blowercontrol 194 which controls the activation of the blower 108. Accordingto this aspect time, temperature, and the cooling rate data are fed backinto the blower control 194, and based on this, data, the blower control194 adjusts the activation of the blower 108 to maintain a prescribedrate of cooling. The blower is turned on until the temperature is cooledto 400° F. or less. Generally, the time the blower is activated isdependent upon numerous factors, including the load in the furnace, andthe process that had been run in the furnace.

Referring now to FIG. 5, a CVI/CVD or heat treatment furnace and coolingsystem is presented that combines the features of FIGS. 3 and 4. Thevarious components previously described in relation to FIGS. 1 and 2 arepresented in FIG. 5, except that furnace 138 having a furnace shell 140is provided. The furnace shell 140 has a center portion 128 disposedbetween two end portions 130 and 132. According to this aspect of theinvention, a closed circuit 142 having cooling gas inlet 118 and 136 isconnected to the furnace 138 at the end portion 130 and the centerportion 128, respectively. Connecting the inlet conduit 136 to thefurnace 138 at the center portion 128 provides a flow of the cooling gasto an area of the furnace 138 that is typically the hottest, whileconnecting the cooling gas inlet 118 to the furnace 138 at the endportion 130 provides a flow of gas to refractory composites 62 disposedbelow the inlet conduit 136. Multiple inlet conduits 136 may beprovided, if so desired. The cooling gas outlet 120 is connected to theother of the end portions 132. Overall cooling of the furnace maythereby be improved relative to the embodiments of FIGS. 3 and 4 withthese two particular embodiments.

Other connections into a furnace may also be utilized as cooling gasinlets or cooling gas outlets. Referring now to FIG. 6, for example, aCVI/CVD or a heat treatment furnace and cooling system is presentedaccording to a further aspect of the invention. The various componentspreviously described in relation to FIG. 5 are presented in FIG. 6. Afurnace 144 is provided having a furnace shell 146 with a center portion128 disposed between two end portions 130 and 132. Furnace 144 comprisesa reactant gas inlet 148 connected to the furnace shell 146 in fluidcommunication with the furnace volume 114. A closed circuit 152 isprovided wherein the cooling gas inlet 118 is connected to the furnaceshell 146 through the reactant gas inlet 148 and is configured toselectively introduce cooling gas into the furnace volume 114 throughthe reactant gas inlet 148. Thus, the cooling gas inlet 118 is in fluidcommunication with the furnace volume 114 through the reactant gas inlet148. At times, reactant gas flow rather than cooling gas flow is desiredthrough the reactant gas inlet 148. Thus, cooling gas is selectivelyintroduced into the furnace volume 114 when such flow is desired. Thisis preferably accomplished by provision of a valve 150 provided in thecooling gas inlet 118 that isolates the reactant gas inlet 148 from thecooling gas inlet 118 when closed. The inlet conduit 136 may be providedand connected to the center portion 128.

Referring now to FIG. 7, a preferred embodiment of the inventioncomprising a furnace 154 and a closed circuit 156 is shown. In furnace154 the heating element 116 comprises a susceptor 158 and an inductioncoil 160 disposed adjacent the susceptor 158, and the cooling element isconfigured to cool the furnace shell 146, which, in this example,comprises a double wall with a space 147 in between filled with coolingwater that is circulated through the heat exchanger 105. The space 147may be separated into multiple sub-spaces with independent cooling waterflow circuits. In addition, induction coils typically comprise amultitude of coil cooling passages 162 integrally formed into theinduction coil 160. Thus, the cooling element may further comprise theinduction coil 160 with integral cooling passages 162 although,according to a preferred embodiment, the cooling is predominantly (ifnot totally) provided by the furnace shell 146.

The susceptor 158 typically comprises a susceptor lid 164 and asusceptor floor 166. The reactant gas inlet 148 passes through thesusceptor floor 166. The cooling gas outlet 120 is disposed beneath thecenter portion 128, and the inlet conduit 136 is connected to the centerportion 128 and passes through the induction coil 160 and susceptor 158.Cooling gas introduced into the cooling gas inlet 118 and inlet conduit136 enters the volume encircled by the susceptor 158 where therefractory composites 62 are disposed. The cooling gas then passes upthrough the susceptor lid 164 (which is typically perforated) and overthe inside surface of the furnace shell 146 and down between the furnaceshell 146 and the induction coil 160, where it is cooled, and thenpasses into the cooling gas outlet 120 and back to the blower 108.Activation of the blower 108 causes cooling gas 106 introduced into thefurnace volume 114 to flow through the blower 108, through the coolinggas inlet 118, over the cooling element (in this example, the furnaceshell 146 and induction coil 160 with cooling passages 162), through thecooling gas outlet 120, and back to the blower 108 in a closed circuit.In this embodiment, the cooling element is embodied in two sub-elementsand serves two purposes. It cools the furnace shell 146 and theinduction coil 160 when the coil is heating the susceptor 158 and,alternatively, cools the cooling gas when the closed circuit 156 isoperated to cool the furnace 154.

Although described in relation to the cooling element being the furnaceshell 146 and/or the induction coil 160 with coil cooling passages 162,any arrangement disposed within the furnace for the purpose of cooling acomponent inside the furnace may be employed to cool the cooling gas,and such arrangements may take a variety of configurations whetheremployed to cool the furnace shell, an induction coil, or otherwise, anyof which are intended to be included within the scope of the invention.Finally, the cooling gas inlet may comprise one or more auxiliaryinlets, such as auxiliary inlet 168 (shown as a dashed line) connectedto the furnace above the center portion 128 in order to provide a flowof cooler gas to the top of the induction coil 160 where hotter gas frominside the susceptor passes over the induction coil 160 in transit tothe cooling gas outlet 120 disposed below the center portion. Othervariations may be employed, as desired, to achieve a particular desiredflow pattern and/or to eliminate hot and/or cold spots. Shut-off valves190 are preferably provided in the auxiliary inlet 168, the inletconduit 136, and the cooling gas outlet 120 that isolate the furnace 154from the rest of the closed circuit during a CVI/CVD or heat treatmentprocess. A shut-off valve 192 is preferably provided in the reactant gasinlet 148 that isolates the reactant gas supply from the closed circuit156 while using the closed circuit cool the furnace 154.

Referring now to FIG. 8, a cross sectional view of an embodiment of theblower 108 is presented, according to a preferred aspect of theinvention, taken along line 8—8 of FIG. 3. The blower 108 comprises ahousing 170 and a drive shaft 172 extending therefrom, and an inert gaspurged dynamic seal 174 between the housing 170 and the drive shaft 172.The housing 170 comprises a main housing 184. A pair of bearingassemblies 186 mounted to the main housing 184 support the drive shaft172. An impeller 188 is attached to the drive shaft 172. The impeller188 may be configured for axial flow, centrifugal flow, or a combinationthereof, as a fan or otherwise. The inert gas purged dynamic seal 174comprises a pair of seals 176 that may be spaced apart and disposedwithin a sealed seal housing 182 that is sealed to housing 170, and aninert gas inlet 178 that introduces inert gas 194 into the space betweenthe seals 180 at a pressure greater than atmospheric pressure. Thecooling gas 106 may be employed as the inert gas 194. Purging the spacebetween the bearings with pressurized inert gas eliminates oxygeningress into the cooling gas within the blower 108 and the closedcircuit through which the blower 108 drives cooling gas. An inert gaspurged dynamic seal 174 may not be necessary or desirable in all aspectsof the invention. Other components, such as view ports, may be inert gassealed with dynamic or static seals, depending on whether moving partsare employed. According to a preferred embodiment of the invention forprocessing high temperature composite materials, the entire closedcircuit is sealed to prevent ingress of oxygen into the closed circuit.Carbon seals have been found to be particularly desired for seals 176 insuch an embodiment. Inert gas purged seals may be employed to minimizeor eliminate ingress of oxygen, when desired.

Referring now to FIG. 9, a cross-sectional view of a high temperaturefurnace 10 is presented, by way of example, that implements variousaspects of the invention. Furnace 10 is configured to be employed with ahigh temperature process. Furnace 10 is generally cylindrical andcomprises a steel shell 12 and a steel lid 14 both formed as doublewalls with a space 13 in between for circulation of cooling water, aspreviously described in relation to FIG. 5. Still referring to FIG. 9,the shell 12 comprises a flange 16 and the lid 14 comprises a matingflange 18 that seals against flange 16 when the lid 14 is installed uponthe shell 12. The shell 12 and lid 14 together define a furnace volume22 that is reduced to vacuum pressure by a steam vacuum generator (notshown) in fluid communication with the vacuum port 20. The shell 12rests upon a multitude of legs 62 (FIG. 9.) The furnace 10 alsocomprises a cylindrical induction coil 24 adjacent a cylindricalsusceptor 26. The induction coil 24 comprises coiled conductors 23encapsulated by electrical insulation 27. During operation, theinduction coil 24 develops an electromagnetic field that couples withthe susceptor 26 and generates heat within the susceptor 26. Theinduction coil 24 may be cooled, typically by integral water passages 25within the coil 24. The susceptor 26 rests upon a susceptor floor 28 andis covered by a susceptor lid 30. A cylindrical insulation wall 32 isdisposed in between the susceptor 26 and the induction coil 24. Lidinsulation layer 34 and floor insulation layer 36 are disposed over thesusceptor lid 30 and beneath the susceptor floor 28, respectively. Thesusceptor floor 28 rests upon the insulation layer 36 which, in turn,rests upon a furnace floor 38. The furnace floor 38 is attached to theshell 12 by a floor support structure 40 that comprises a multitude ofvertical web structures 42. A reactant gas is supplied to the furnace 10by a main gas supply line 44. A multitude of individual gas supply lines46 are connected in fluid communication with a multitude of gas ports 48that pass through the furnace shell 12. A multitude of flexible gassupply lines 50 are connected in fluid communication with the gas ports48 and a multitude of gas inlets 52 that pass through holes 54 in thefurnace floor 38, the floor insulation layer 36, and the susceptor floor28. A gas preheater 56 rests on the susceptor floor 28 and comprises amultitude of stacked perforated plates 58 that are spaced from oneanother by a spacing structure 60. Each plate 58 is provided with anarray of perforations that are horizontally shifted from the array ofperforations of the adjacent plate 58. This causes the reactant gas topass back and forth through the plates, which diffuses the reactant gaswithin the preheater 56 and increases convective heat transfer to thegas from the perforated plates 58. A multitude of refractory composites62, for example brake disks, are stacked within the furnace 10 insidethe susceptor 26 on fixtures (not shown for clarity). Suitable fixturesare well known in the art.

Still referring to FIG. 9, the susceptor 26 is configured as acylindrical wall 26 having a center portion 66 disposed between two endportions 68 and 70. An inlet conduit 72 enters the furnace 10. Thecenter portion 66 has a hole 74 therein with the inlet conduit 72entering the hole 74 and being configured to introduce cooling gaswithin the cylindrical wall 26 at the hole 74. An insulating bushing 76may be disposed within the hole 74 mating with the cylindrical wall 26and the inlet conduit 72. In passing through the hole 74, the inletconduit 72 extends through the induction coil 24 and the insulation wall32. The inlet conduit 72 is preferably made from an insulating materialand mates with a steel conduit 73 that is welded to the furnace 78. Apliant gasket 80 is disposed between the inlet conduit 72 and the steelconduit 73, which permits the inlet conduit 72 to move relative to thesteel conduit 73 as the furnace 10 heats up and cools down whilemaintaining a seal. If the insulating bushing 76 is made from a porousinsulating material, a bushing seal layer 82 may be bonded to thesurface that would otherwise be exposed to reactant gas. The insidediameter of the inlet conduit 72 is preferably covered with animpervious sheet if the inlet conduit 72 is made from a porousinsulating material. According to a preferred embodiment for CVI/CVDdepositing a pyrolytic carbon matrix within carbon fiber porousstructures for aircraft brake disks, the furnace 154 of FIG. 7 isconfigured as furnace 100 of FIG. 9, preferably with the auxiliary inlet168. According to a certain embodiment, the inlet conduit 72 ismanufactured from porous carbon, such as Porous Carbon 60 material,available from UCAR Carbon Company Inc., United States of America. Theinsulating bushing 76 is a rigid felt, such as Calcarb CBCF material,available from Calcarb, Ltd., Scotland, or Fibergraph® material,available from SIGRI Polycarbon, Inc., United States of America. Thebushing seal layer 82, pliant gasket 80, and impervious layer lininginside the inlet conduit 72 are a graphite foil, such as Grafoil®material, also available from UCAR Carbon Company Inc. Calgraph® brandgraphite foil may also be employed, also available from SIGRIPolycarbon, Inc.

A method of cooling a furnace initially at CVI/CVD process temperatures(on the order of 1800° F.) proceeds as follows. Valve 192 is closed andthe furnace volume 22 inside the furnace is back-filled from vacuum(about 10 torr) to atmospheric pressure by flowing on the order of 275Standard Cubic Feet/Hour (“SCFH”) nitrogen, 200 SCFH helium, and 75 SCFHargon. When the pressure of furnace volume 22 reaches on the order ofatmospheric pressure, all gas flows are terminated and the valves 190are opened. The oxygen content analyzer sensor 110 (FIG. 1) is activatedalong with the fan shaft seal purge. The blower 108 at a speed of 25 Hz(the blower is rated at 800 CFM at 60 Hz) is activated and the oxygenlevel of the cooling gas 106 is monitored and maintained at less than orequal to 100 ppm. Oxygen levels typically remain steady in the range of40-100 ppm, and should reach that range after 15-30 minutes. If oxygenlevels become too high, i.e., over 100 ppm, the entire system is shutoff. Upon temperature inside the furnace decreasing to on the order of1050° F. the fan speed is increased to 30 Hz and a flow of 30 SCFHhelium is initiated and subsequently terminated after a period ofapproximately six hours (the vessel is pressure relieved to avoidpositive pressure above atmospheric). Upon temperature reaching 750° F.,the fan speed is increased to 35 Hz and a flow of 30 SCFH of helium isagain initiated for another period of approximately six hours andthereafter terminated. When the temperature inside the furnace isdecreased to a final temperature of 600° F. or less, the furnace lid maybe removed and the cooling system deactivated. Alternatively, thecooling system may be left running in order to circulate atmospheric airthrough the furnace. Increasing fan speed and helium flow rate as thefurnace cools increases the cooling rate and allows approximation of alinear cool-down (rather than asymptotic) from the initial temperatureto the final temperature. This method is particularly useful for coolingcarbon/carbon composite brake disks from CVI/CVD processing temperature.Alternatively, instead of helium, nitrogen can be used for thecool-down.

A method of cooling a furnace initially at a refractory composite heattreatment temperature (on the order of 3400° F.) proceeds similarly tothe process just described with the following exceptions. The furnace isback-filled with a gas mixture that is ¾ argon and ¼ helium since thesegases are stable at that initial temperature. Less helium is used atgreater temperatures in order to prevent cooling at too fast of a rate,which may damage components inside the furnace, for example theinduction coil and/or the refractory composite structures being heattreated. When the furnace temperature reaches on the order of 1850° F.,a 30 SCFH flow of helium is initiated. Subsequently, additional heliumand higher fan speeds are enacted as previously described.

Referring now to FIG. 10, a furnace 10 according to a further aspect ofthe invention is presented that is similar to furnace 10 of FIG. 9except the cooling gas inlet through the side of the furnace is replacedby a similar inlet that enters the furnace from the bottom and passes upthrough the center of the gas preheater 56. The inlet conduit 72 may besplit into multiple inlets if desired. This furnace may be implementedaccording to the embodiment of FIG. 5.

Although the invention has been described and illustrated with referenceto specific illustrative embodiments thereof, it is not intended that heinvention be limited to those illustrative embodiments. Those skilled inthe art will recognize that variations and modifications can be madewithout departing from the true scope and spirit of the invention asdefined by the claims that follow. It is therefore intended to includewithin the invention all such variations and modifications as fallwithin the scope of the appended claims and equivalents thereof.

What is claimed is:
 1. A furnace configured to process refractorycomposites and a cooling system, comprising: a furnace shell thatdefines a furnace volume, said furnace shell having a center portion; areactant gas inlet connected to said furnace shell in fluidcommunication with said furnace volume; a susceptor disposed within saidfurnace shell; an induction coil disposed within said furnace shelladjacent said susceptor, said induction coil comprising a coolingelement; an inlet conduit connected to said furnace shell at said centerportion and through said reactant gas inlet, said inlet conduit being influid communication with said furnace volume; an outlet conduitconnected to said furnace shell in fluid communication with said furnacevolume; a cooling gas supply configured to selectively introduce acooling gas into said furnace volume; a blower connected to said inletconduit and said outlet conduit in fluid communication therewith,wherein activation of said blower causes cooling gas introduced intosaid furnace volume to flow through said blower, through said inletconduit, over said cooling element, through said outlet conduit, andback to said blower in a closed circuit, and a blower control whichcontrols the activation of said blower, wherein time, temperature, andcooling rate data are fed back into said blower control, and said blowercontrol adjusts the activation of said blower to maintain a prescribedrate of cooling.
 2. A furnace configured to process refractorycomposites and a cooling system therefore, comprising: a furnace shellthat defines a furnace volume; a heater disposed within said furnaceshell; a cooling element disposed within said furnace shell; an inletconduit connected to said furnace shell in fluid communication with saidfurnace volume; an outlet conduit connected to said furnace shell influid communication with said furnace volume; a cooling gas supplyconfigured to selectively introduce a cooling gas into said furnacevolume; a blower connected to said inlet conduit and said outlet conduitin fluid communication therewith, wherein activation of said blowercauses cooling gas introduced into said furnace volume to flow throughsaid blower, through said inlet conduit, over said cooling element,through said outlet conduit, and back to said blower in a closedcircuit, wherein said blower comprises a housing and a drive shaftextending therefrom, and an inert gas purged dynamic seal between saidhousing and said drive shaft, and a blower control which controls theactivation of said blower, wherein time, temperature, and cooling ratedata are fed back into said blower control, and said blower controladjusts the activation of said blower to maintain a prescribed rate ofcooling.
 3. A furnace configured to process refractory composites and acooling system, comprising: a furnace shell that defines a furnacevolume, said furnace shell having a center portion; a reactant gas inletconnected to said furnace shell in fluid communication with said furnacevolume; a susceptor disposed within said furnace shell; an inductioncoil disposed within said furnace shell adjacent said susceptor, saidinduction coil comprising a cooling element; an inlet conduit connectedto said furnace shell at said center portion and through said reactantgas inlet, said inlet conduit being in fluid communication with saidfurnace volume; an outlet conduit connected to said furnace shell influid communication with said furnace volume; a cooling gas supplyconfigured to selectively introduce a cooling gas into said furnacevolume, a blower connected to said inlet conduit and said outlet conduitin fluid communication therewith, wherein activation of said blowercauses cooling gas introduced into said furnace volume to flow throughsaid blower, through said inlet conduit, over said cooling element,through said outlet conduit, and back to said blower in a closedcircuit, wherein said blower comprises a housing and a drive shaftextending therefrom, and an inert gas purged dynamic seal between saidhousing and said drive shaft, and a blower control which controls theactivation of said blower, wherein time, temperature, and cooling ratedata are fed back into said blower control, and said blower controladjusts the activation of said blower to maintain a prescribed rate ofcooling.
 4. A furnace configured to process refractory composites and acooling system therefore, comprising: a furnace shell that defines afurnace volume; a heater disposed within said furnace shell; a coolingelement disposed within said furnace shell; an inlet conduit connectedto said furnace shell in fluid communication with said furnace volume;an outlet conduit connected to said furnace shell in fluid communicationwith said furnace volume; a cooling gas supply configured to selectivelyintroduce a cooling gas into said furnace volume; a blower connected tosaid inlet conduit and said outlet conduit in fluid communicationtherewith, wherein activation of said blower causes cooling gasintroduced into said furnace volume to flow through said blower, throughsaid inlet conduit, over said cooling element, through said outletconduit, and back to said blower in a closed circuit, and a blowercontrol which controls the activation of said blower, wherein time,temperature, and cooling rate data are fed back into said blowercontrol, and said blower control adjusts the activation of said blowerto maintain a prescribed rate of cooling, wherein said furnace shell hasa center portion, said inlet conduit being connected to said furnace atsaid center portion.
 5. A furnace configured to process refractorycomposites and a cooling system therefore, comprising: a furnace shellthat defines a furnace volume; a heater disposed within said furnaceshell; a cooling element disposed within said furnace shell; an inletconduit connected to said furnace shell in fluid communication with saidfurnace volume; an outlet conduit connected to said furnace shell influid communication with said furnace volume; a cooling gas supplyconfigured to selectively introduce a cooling gas into said furnacevolume; a blower connected to said inlet conduit and said outlet conduitin fluid communication therewith, wherein activation of said blowercauses cooling gas introduced into said furnace volume to flow throughsaid blower, through said inlet conduit, over said cooling element,through said outlet conduit, and back to said blower in a closedcircuit, and a blower control which controls the activation of saidblower, wherein time, temperature, and cooling rate data are fed backinto said blower control, and said blower control adjusts the activationof said blower to maintain a prescribed rate of cooling, wherein saidfurnace shell has a center portion disposed between two end portions,said inlet conduit being connected to said center portion and one ofsaid end portions, said outlet conduit being connected to the other ofsaid end portions.